Embodiments of the invention relates to a wind turbine. Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
Gearboxes form one of the most maintenance-intensive components of the wind turbine. They need to be inspected regularly and do not always fulfil their expected service life; the gearbox or some of its parts sometimes need to be replaced prematurely. This is due to the high loads and fluctuating loads to which a gearbox is subjected. Particularly, the bending loads on the blades, which may be transmitted through the rotor shaft to the gearbox are damaging.
The cause of the transmission of the bending loads and deformations from the blades and hub to the generator lies in the wind turbine configuration. In most conventional wind turbines, the rotor hub is mounted on one end of the rotor shaft. The rotor shaft is rotatably mounted in a support structure within the nacelle on top of the wind turbine tower. The rotor thus forms an overhanging structure that transmits torque, but additionally transmits cyclical bending loads due to the loads on the blades and the weight of the hub and blades. These bending loads are transmitted either to the generator (in the case of direct drive turbines) causing air gap variations or to the gearbox causing fluctuating loads in the gearbox.
In order to solve this problem, it is known from e.g. ES 2 163 362 to provide a wind turbine tower with a forward extending frame. The rotor hub with its plurality of blades is mounted and can rotate upon the frame; the rotor hub is coupled to a rotor shaft located within the frame. Such a wind turbine has been schematically indicated in FIG. 1. In FIG. 1, a wind turbine 100 comprises a hub 110, which is rotatably mounted upon frame 170, at a distal end of the frame. Frame 170 is mounted upon tower 180. A coupling element 120 couples rotor shaft 130 to hub 110. The rotation of rotor shaft 130 is transformed with a gearbox 140 to a fast rotation of output shaft 150 which drives generator 160.
In a prior art coupling, a center piece is mounted on a rotor shaft with a shrink disc. Along the circumference of the annular rim of the center piece a plurality of holes is provided. Bolts provided in elastic bushings are used to connect the center piece to the hub. The elastic bushings make the coupling more flexible in the longitudinal direction of the rotor shaft.
With this kind of configuration comprising a hub mounted on a frame, the loads due to the weight of hub and blades are transmitted more directly via the frame to the tower, whereas the rotor shaft transmits mainly torque to the gearbox (and/or generator), thus avoiding to a certain extent the undesired loads and deformations in the drive train. This represents an improvement with respect to other prior art wind turbines, but the transmission of bending loads from the blades to the rotor shaft, (and through the rotor shaft to the gearbox) cannot be avoided entirely.
Furthermore, mounting the coupling with the plurality of bolts is a cumbersome, and time consuming and therefore expensive task. Disassembling the coupling for maintenance, inspection or repair is of course just as cumbersome. It is furthermore important in this configuration that the rotor shaft be perfectly aligned because a misalignment leads to stresses in the rotor shaft, the coupling and the hub. These stresses may lead to e.g. fatigue problems. The installation process is thus further complicated because of the need to perfectly align the rotor shaft with respect to the hub.
Also, having such a quantity of bolts and flexible elements (e.g. approximately 30 each per wind turbine) significantly raises the part count, which may complicate logistics.
There thus still exists a need for a wind turbine in which the transmission of bending loads from the blades to the rotor shaft is further reduced. There also exists a need for a wind turbine with a reduced part count and which is easier to install and maintain.